1. Field of the Invention
This invention relates to a catalyst for the emission control of aldehyde which efficiently controls the emission of aldehyde as a harmful ingredient in an exhaust gas discharged from an internal engine for automobile or the like using alcohol or alcohol-containing fuel.
2. Description of the Related Art
Heretofore, there have been proposed various catalysts for the emission control of hydrocarbon (HC), carbon monooxide (CO) and nitrogen oxide (NOx) in the exhaust gas discharged from the internal engine. Among them, the use of noble metal such as platinum (Pt) or the like is possible to simultaneously remove out the above three ingredients. In case of using alcohol fuel, aldehydes are further produced through partial oxidation of alcohol in the internal engine, so that it is required to control the emission of aldehydes in the exhaust gas.
However, the conventional noble metal type catalysts possess both oxidative action of alcohol and oxidative action of aldehyde. Therefore, when the exhaust gas is purified by this catalyst, partial oxidation reaction of alcohol not combusted in the internal engine is caused to produce aldehyde, so that the exhaust gas passed through the catalyst still contains aldehyde and hence the conventional catalyst can not be applied to the emission control of aldehyde as it is. Even if it is intended to improve the performance of the three-way catalyst by increasing the amount of noble metal carried per the catalyst, the above problem can not fundamentally solved.
For the purpose of solving the problem, there are proposed catalysts in which silver (Ag) or the like is added to palladium (Pd) as a noble metal to remove aldehyde together with alcohol as disclosed in, for example, Japanese patent laid open No. 62-129129. In this case, it is attempted to improve the catalyst effect by the co-existing action of Pd and Ag.
However, the latter catalysts have problems as mentioned below:
(1) Since the noble metal and silver are fundamentally coexistent to remove aldehyde, the catalytic activity largely differs in accordance with the carrying way and dispersibility; PA1 (2) The formation of aldehyde through partial oxidation of alcohol can not completely be controlled due to the use of the noble metal; and PA1 (3) Since the dispersibility of the active ingredient is important, there is a fear of degrading the catalyst effect during the use, and also there is the rebounding of other ingredient to the catalyst effect due to the alloying or the like.